Air masses and fronts Textbook, Chapter 9

DiscussionHow do precipitation systems form?

How does this precipitation form?

mountains

How does this precip form?

How does this storm form? let’s revisit the lifting mechanisms …

Five Mechanisms 1. Convective ascent Cumulus clouds … spontaneous buoyant

The 4 other mechanisms are forced ascent … (a) over a mountain (b) or a front

(c) Convergence into surface low

(d) Uplift forced by jet stream Upper-level divergence

Areas of jet-stream driven ascent

Cloud & precipitation formation mechanisms Buoyant ascent [bubble ascent] Forced ascent [layer ascent] a)Orographic b)Frontal c)Low-level convergence (friction) d)Upper-level divergence (jet stream) Many clouds are the result of several simultaneous processes.

1. Buoyant ascent

We’ve talked about stability and parcel ascent (Chapter 4, Lab 7). Remember that in a conditionally unstable atmosphere, a cloud will continue to rise as long as its core temperature exceeds that of the surroundings 1. Buoyant ascent

cumulus growth stable environmental lapse rate stable conditionally unstable stable condi- tionally unstable moist dry Cumulus humilis Cumulus congestus Cumulonimbus

Lifting up a mountain may cause destabilization, …which is why Cu clouds first form there

Cloud & precipitation formation mechanisms Buoyant ascent [bubble ascent] Forced ascent [layer ascent] a)Orographic b)Frontal c)Low-level convergence (friction) d)Upper-level divergence (jet stream) Many clouds are the result of several simultaneous processes.

(a) Orographic clouds Altocumulus lenticularis, Greenland

Quantifying the Foehn effect The Foehn or Chinook effect (web link)

Foehn Effect -4ºC 4ºC 12ºC environmental temperature windward slopeleeward slope T T d 20ºC , 0 -2, -2 4, 4 10, 10 rainshadow e.g, Rocky Mt. Chinook winds Higher temperatures on the leeward side of mountains occur because latent heat is released (condensation) and converted into sensible heat.

Chinook wall cloud

TTdTd height of the mountain

Pop quiz: when a good westerly wind blows and heavy snowfall occurs over the Cascades, then in Yakima on the east side it will be ……. than in Seattle on the west side. A: sunnier but cooler B: sunnier, warmer, drier, and windier C: sunnier, cooler, more damp, and calmer D: cloudier, cooler, more damp, and windier. Seattle Yakima

Cloud & precipitation formation mechanisms Buoyant ascent [bubble ascent] Forced ascent [layer ascent] a)Orographic b)Frontal c)Low-level convergence (friction) d)Upper-level divergence (jet stream)

(b) Uplift along fronts A front is a sudden transition between airmasses

Fronts separate airmasses

continental maritime cP cT mP mT Types of airmasses polar tropical

Fig. 9.3

Fig. 9.4

Air masses have distinct lapse rates, and therefore also distinct cloud and precipitation characteristics

Pop quiz: what airmass is this ? cP cT mP mT

Pop quiz: what airmass is this ? cP cT mP mT

Pop quiz: what airmass is this ? cP cT mP mT Hint: this happens in winter

Warm fronts The cold air recedes …

Warm front clouds Where is the cold air??

Fig. 9.16

Cold fronts

Cold front clouds light rain or dry (1)stable air ahead of cold front (2) unstable air ahead of cold front

Fig. 9.14

Occluded fronts Stage IStage II

Cross section through an occluded front

Fig (western US) (eastern US) Cold occlusion Warm occlusion

Stationary fronts Current analysis

3D view of fronts during the evolution of a frontal disturbance

drylines Thunderstorms, sometimes severe ones, can be triggered along the dryline.

Superstorm of 12 March 1993 Locate the front(s)

Case study: winter storm of Dec 2000

Cloud & precipitation formation mechanisms Buoyant ascent [bubble ascent] Forced ascent [layer ascent] a)Orographic b)Frontal c)Low-level convergence (friction) d)Upper-level divergence (jet stream)

(c) Low-level convergence into lows or trofs 1)Tropical lows (hurricanes) extract their energy from latent heat release 2)Mid-latitude lows extract most of their energy from the jet stream

LOW-LEVEL CONVERGENCE INTO A TROF Tropical cyclogenesis

Cross section thru a tropical cyclone LOW-LEVEL CONVERGENCE

Check out some hurricane animations

Tropical cyclones have different names in different oceans There spin is always cyclonic, i.e. ccw in the north and cw in the south

Where do hurricanes form? –Away from the equator –Where the SST is at least 80  F August SST

Hurricane Effects: Wind: causes most damage, but limited mortality. Storm surge: a historical killer, now mitigated by evacuation. Rain and Inland Flooding: take more lives than wind, but do less damage.

coastal Hurricane mortality and damage, United States Hurricanes are covered in the textbook in Chapter 8

Lows, and convergence, uplift, clouds, and precipitation, also occur at higher latitudes. What energizes them?

Answer: the jet stream !! Buoyant ascent [bubble ascent] Forced ascent [layer ascent] a)Orographic b)Frontal c)Low-level convergence (friction) d)Upper-level divergence (jet stream) THIS WILL BE COVERED IN CHAPTER 10 End of Chapter 9